Re: [PATCH] KEYS: trusted: Use get_random-fallback for TPM

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Sun, 2025-12-14 at 23:32 +0200, Jarkko Sakkinen wrote:
> 1. tpm2_get_random() is costly when TCG_TPM2_HMAC is enabled and thus
> its
>    use should be pooled rather than directly used. This both reduces
>    latency and improves its predictability.
> 
> 2. Linux is better off overall if every subsystem uses the same
> source for
>    the random bistream as the de-facto choice, unless *force majeure*
>    reasons point to some other direction.
> 
> In the case, of TPM there is no reason for trusted keys to invoke TPM
> directly.

That assertion isn't correct: you seem to have forgotten we had this
argument six or seven years ago, but even that was a reprise of an even
earlier one.  Lore doesn't go back far enough for the intermediate one
on the tpm list, but the original was cc'd to lkml:

https://lore.kernel.org/all/1378920168.26698.64.camel@localhost/

The decision then was to use the same random source as the key
protection.  Unfortunately most of the active participants have moved
on from IBM and I don't have their current email addresses, but the
bottom line is there were good reasons to do trusted keys this way that
your assertions above don't overcome.  I'm not saying we shouldn't
reconsider the situation, but we need a reasoned debate rather than
simply doing it by fiat.

Regards,

James